The present invention generally relates to image pickup devices, and more particularly to an image pickup device having a high resolution and applicable to a movie camera for taking moving pictures, a still camera for taking still pictures and the like.
A video signal which is obtained by picking up an optical image of an object on an image pickup device can be subjected to a video signal processing with ease. The video signal processing includes editing, trimming and the like. In addition, such a video signal can be recorded and reproduced on and from a reversible recording medium with ease, where a recorded signal is erasable for repeated recording and playback.
On the other hand, in the image pickup device which is conventionally used to generate the video signal, an image formation of the optical image of the object is made on a photoelectric conversion part of an imaging element (photoelectric transducer) by use of an imaging lens. The optical image of the object is converted into an electrical image signal in the photoelectric conversion part of the imaging element. The electrical image signal is outputted sequentially on the time base as the video signal. As is well known, various kinds of pickup tubes and various kinds of solid state image sensors are used as the imaging element of the image pickup device.
Recently, there is much demand to reproduce a picture of both picture quality and resolution considerably higher than those of the currently utilized television systems. In accordance with such a demand, the so-called extended definition television (EDTV) and high definition television (HDTV) systems have been proposed for the purpose.
In order to obtain a picture of high picture quality and resolution, the image pickup device must be able to generate a video signal which enables a reproduction of the picture of high picture quality and resolution.
In the case of the image pickup device which uses the pickup tube as the imaging element, it is possible to consider reducing a diameter of an electron beam or increase a target area for the purpose of generating the video signal which enables the reproduction of the picture of high picture quality and resolution.
However, there is a limitation for reducing a diameter of an electron beam due to the performance of an electron gun, the structure of a convergence system and the like. For this reason, the realization of the high resolution by reducing the electron beam diameter cannot be expected.
On the other hand, since a target capacitance of the pickup tube increases with the target area, high-frequency components in the output signal of the pickup tube become degraded as the target capacitance increases and a deterioration in a signal-to-noise (S/N) ratio of the output signal of the pickup tube becomes untolerable. Furthermore, in the case of the image pickup device for use in taking moving pictures, a frequency range of the video signal becomes several tens of MHz to several hundreds of MHz and causes problems in view of the S/N ratio. Hence, the realization of the high resolution by increasing the target area seems not practical.
For these reasons, it is extremely difficult to generate a video signal which enables the reproduction of the picture of high picture quality and resolution from the image pickup device using the pickup tube as the imaging element.
On the other hand, in the case of the image pickup device which uses the solid state image sensor as the imaging element, it is conceivable to use a solid state image sensor having a large number of picture elements (pixels) for the purpose of generating the video signal which enables the reproduction of the picture of high picture quality and resolution. But a clock signal which drives the solid state image sensor having the large number of picture elements naturally requires to be a high frequency. For example, in the case of the image pickup device for use in taking moving pictures, the clock signal for driving the solid state image sensor has a high frequency in the order of several hundreds of MHz. In addition, an electrostatic capacitance of the associated circuit which is driven increases as the number of picture elements increases. Since an upper limit frequency of the clock signal for driving the solid state image sensor is presently believed to be approximately 20 MHz, it is virtually impossible from the practical point of view to use the solid state image sensor as the imaging element for the purpose of generating the video signal which enables the reproduction of the picture of high picture quality and resolution.
Therefore, in the conventional image pickup device, it is impossible to satisfactorily generate a video signal which enables a reproduced picture of high picture quality and resolution, due to the limitations of the imaging element which is used. Thus, there is much demand to realize an image pickup device which has no such limitations thus enables to generate a video signal of the reproduced picture of high picture quality and resolution.